This invention relates generally to semiconductor devices and manufacturing methods and, more particularly, to compensation doping of semiconductor materials formed by vapor phase epitaxy.
As is known in the art, semiconductor devices such as field effect transistors and monolithic microwave integrated circuits are often employed to amplify or process radio frequency signals. For example, a field effect transistor is often employed to convert D.C. power to radio frequency power by feeding an R.F. voltage signal to a gate electrode of the field effect transistor to thereby control conductivity of an underlying drain-source channel of the field effect transistor. Radio frequency performance is dependent upon the quality of the crystalline structure of the semiconductor layers used to form the field effect transistor. As is also known in the art, Group III-V semiconductor material systems such as the system employing gallium arsenide (GaAs) are often used to fabricate field effect transistors for amplifying or for converting D.C. power to radio frequency power.
One technique used in the prior art to provide semiconductor layers for field effect transistors is to grow such layers directly over a substrate comprising the Group III-V material, for example, gallium arsenide. The substrate generally is prepared with a relatively high bulk resistivity, typically in the range of 10.sup.7 -10.sup.8 ohms-cm. This relatively high resistivity is required to provide the field effect transistors with relatively low leakage currents. The leakage current of the field effect transistor affects the field effect transistor's R.F. performance since the leakage current cannot readily be controlled or modulated by the R.F. voltage signal applied to the gate electrode.
Generally, two methods are employed in the prior art to prepare substrates of gallium arsenide having a relatively high bulk resistivity. One method is the so-called "Horizontal Bridgeman Technique" which involves the steps of introducing elemental gallium and arsenic into a quartz reaction vessel disposed in a furnace at an elevated temperature, reacting the gallium and arsenic to form a gallium arsenide melt, and slowly withdrawing the sealed vessel from the furnace to form a bar of the material having a crystalline structure. The bar is sliced into substrates which are then lapped and polished. The resistivity of these substrates is generally lowered because residual donor ions originating from the quartz reaction vessel are present in the crystal structure. Accordingly, this method generally requires doping with a compensating acceptor material such as chromium. With a second method, the so-called "Czochralski Technique", a seed crystal is slowly withdrawn from a gallium arsenide melt in a controlled atmosphere. This technique is particularly useful in providing relatively large circular substrates of gallium arsenide. Several variations of this technique have been developed including the more widely used Liquid Encapsulated Czochralski Technique where a seed crystal is pulled through a layer of melted boron oxide which acts as an encapsulant to assist in prevention of arsenic from leaving the melt, a problem generally common to the Czochralski technique.
A problem common to both techniques, particularly the Czochralski, is that extra or interstitial arsenic atoms result in stoichiometric defects generally denoted in the art as "EL2". As shown in FIG. 1, these defects may become ionized in the presence of an electron current flux and produce an ionized center (fixed positive charge) and a free electron in the conduction band. These ionized sites and free electrons are partially compensated to maintain semi-insulating characteristics by chromium doping described above or by the presence of chemical contaminants in the gallium arsenide melts which act as electron acceptors. Chromium as well as these contaminants in the presence of an electron current flux act as electrically active impurities by accepting an electron from the valance band which creates a hole current in the valance band and also an ionized center (fixed negative charge). The balance of fixed charges and currents provides the high resistivity characteristics.
However, the crystalline quality of the substrates fabricated by either method is generally not suitable for fabrication of high performance field effect transistors directly thereover because inside of the crystal structure close to the surface of the substrate, unwanted crystalline defects such as hole and electron traps are present which can degrade the electrical properties of the device. These traps can become ionized sites either accepting or emitting an electron. During operation of a field effect transistor, the electric field created by ionization of these traps restricts the flow of electrons in the channel with a concomitant loss in power, an effect generally known in the art as "backgating".
One solution known in the art is to provide a buffer layer comprising an epitaxially grown crystalline layer over the substrate between active regions of the semiconductor device and the substrate. The buffer layer should provide a layer having a high crystalline quality, and high resistivity to shield the active regions of the field effect transistor from crystalline defects in the substrate.
One method suggested in the art is to grow a buffer layer using chromium as a compensating dopant. Chromium or a similar "deep level" acceptor accepts electrons having an energy level intermediate the crystal's valence band level and the crystal's conduction band providing an ionized site or fixed negative charge and a hole current in the valence band of the crystal. As previously mentioned, contaminants such as Si provided by the growth apparatus are electron donors. When ionized, atoms such as Si provide an electron current flow in the conduction band of the crystal and an ionized site or fixed positive charge. It is an object, therefore, when compensating with chromium to provide a hole current flow in the valence band of the crystal and hence a fixed negative charge to compensate for the electron flow in the conduction band of the crystal and fixed positive charge.
Several methods have been suggested in the art for growing buffer layers doped with a deep level acceptor such as chromium. In one of these methods, vapors, for example, from either CrO.sub.2 Cl.sub.2, Cr(CO).sub.6 or Cr.sub.2 O.sub.3 sources are decomposed to provide chromium. More particularly, these vapors are directed into a reactor tube apparatus where they react to provide elemental chromium which is incorporated in the crystal structure of the gallium arsenide buffer layer. One problem with this technique is that the reactions, for example the CrO.sub.2 Cl.sub.2 reaction, are highly endothermic. Accordingly, large deposits of chromium are generally left on the walls of the reactor tube. Because of the presence of Cr on the walls of the reactor, it is therefore difficult to control the concentration of chromium in the buffer layers. Furthermore, since the chromium remains in the reactor tube, it is generally very difficult to automatically start growing the active regions of the semiconductor without having the chromium dopant present in high concentrations in the active regions. Doping of the active regions with chromium is highly undesirable, since chromium being a deep level acceptor will become ionized in response to an electron current flux. This will result in a fixed negative charge in the active layer which will repel electrons in the active layer thereby constricting the channel and hence reducing current density and device performance.
A second method used for chromium doping involves etching a source of elemental chromium. In a typical vapor phase epitaxial apparatus, the quartz reactor tube is disposed in a furnace and has mounted therein a substrate upon which is grown the epitaxial layer. The quartz reactor tube has a first end which is fed via a pair of lines, a first line generally denoted as "the growth line" and a second line downstream from the growth line generally denoted as "the etching line." The growth line and etching line are each fed from a reactant source of, for example, arsenic trichloride. The arsenic trichloride is provided, via a growth bubbler, in a vapor stream comprised of hydrogen. From the growth line, arsenic trichloride vapors are directed over a source of the material to be grown, in this case gallium arsenide. The arsenic trichloride reacts with the gallium arsenide source providing a vapor stream comprising gallium chloride and arsenic. From the etching line, arsenic trichloride vapors are directed over the source of chromium which is disposed in the same reactor tube and same temperature zone of the tube and separated from the gallium arsenide source. This arsenic trichloride vapor also reacts with the chromium to provide chromium chloride as the dopant precursor. While the chromium doping of an epitaxial layer is possible using this technique, there are nevertheless several problems with this technique. With this method as mentioned, two vapor flows are directed towards the substrate. The first flow is the growth flow comprised of the gallium chloride and arsenic and the second flow is the flow of the dopant as chromium chloride (CrCl.sub.2). The chromium chloride is provided by the reactions over the Cr source as: EQU 2 AsCl.sub.3 +3H.sub.2 .fwdarw.1/2As.sub.4 +6HCl (1) EQU Cr+2HCl.fwdarw.CrCl.sub.2 +H.sub.2 ( 2A)
whereas the gallium arsenide precursor components in the vapor stream are provided by the reaction over the GaAs solid source as: EQU GaAs+HCl.fwdarw.GaCl.sub.x 1/4As.sub.4 +H.sub.2 ( 3A)
where x is generally 1, 2, 3 and/or 4. In the growth region over the substrate where the temperature of the tube is lower, these reactions reverse direction (due to lower temperature over the substrate) to provide the doped epitaxial material in accordance with the following reactions: EQU CrCl.sub.2 +H.sub.2 .fwdarw.Cr+2HCl (2B) EQU GaCl.sub.x +1/4As.sub.4 +H.sub.2 .fwdarw.GaAs+2HCl (3B)
However, at the substrate because of the decomposition of CrCl.sub.2 +H.sub.2 into Cr and HCl, the concentration of HCl ([HCl]) in the composite vapor stream is higher in comparison to the [HCl] without Cr doping. Since the [HCl] is increased, reaction (3B) will not proceed as efficiently and will have a tendency to reverse direction and proceed as reaction (3A) even at the lower reaction temperature. That is, the reaction will tend to produce GaCl.sub.x and As.sub.4 by etching of the GaAs substrate due to the excess [HCl]. In other words, the rate at which GaAs is deposited out of the stream and onto the substrate will be reduced and, further, if the increase in [HCl] is large enough, the reaction could become a net etching reaction in which GaAs is removed from the substrate.
Thus, the extra HCl flow in the vapor flow over the substrate slows the growth rate reaction because the increase in [HCl] shifts the reaction equilibrium of reaction [3B] towards the right to establish a new equilibrium. This reduced growth rate causes two problems. The first problem is that the time required to grow the epitaxial layers increases resulting in a concomitant increase in cost. Secondly, since the growth rate is relatively slow, impurities present in the substrate have a longer period of time over which to out-diffuse from the substrate into the buffer layer. This is particularly true with so-called "fast diffusing impurities" such as iron and copper. Diffusion of these impurities into a compensated buffer layer provides a concomitant reduction in the resistivity of the buffer layer and is therefore undesirable. One of the objectives in growing a chromium doped buffer layer is to grow the buffer layer sufficiently fast with the dopant so that the so-called "fast diffusion impurities" like iron and copper do not have a sufficient amount of time to out-diffuse from the substrate. By providing the increased concentration of hydrogen chloride in the vapor stream, slowing or even halting of the growth of GaAs results. This provides additional time for the impurities from the substrate to diffuse into the buffer layer.
A problem with using chromium as a compensating dopant is that chromium being a so-called "deep level acceptor", that is, having an electron energy level intermediate the crystal conduction and valence bands as shown in FIG. 1, may provide in response to a large, high frequency changing electron current flux, a fixed negative charge layer which may repel electrons in the active layer of the field effect transistor, particularly, power field effect transistors. This fixed charge results from the rate of recombination of electrons and holes between the valence band and the "deep level acceptor" chromium being lower than the rate of change of the injected electron current flux in the conduction band. This problem with chromium is particularly important or evident where the chromium concentration is not readily controlled.
Further, it is also believed that with the current techniques Cr has a tendency to out-diffuse from the buffer layer into active layers of the field effect transistor when the concentration of Cr is not readily controlled. This diffusion is particularly undesirable because it results in a decrease in electron mobility which affects current density and device performance as mentioned previously. Further, the fixed negative charge layer may also occur further degrading device performance as described above.
A further problem common to most techniques is the difficulty in growing a buffer layer having a uniform concentration of chromium over the entire substrate surface.
Accordingly, although chromium doping is known as a compensation technique for increasing the resistivity of buffer layers, there are inherent problems associated with the known techniques which limits the usefulness of chromium doping for epitaxial buffer layers. With either technique, it is generally difficult to control the concentration of chromium during doping of the buffer layer and, in particular, it is difficult to stop doping of the chromium when changing from buffer layer growth to active layer growth. This may result in out-diffusion of Cr from the buffer layer or doping of active layers with Cr through incorporation of residual Cr in the reactor tube. Accordingly, a fixed net charge layer may be provided adjacent the active layer during operation of the field effect transistor. Further, with the etching technique of a solid elemental Cr source, the growth rate of the gallium arsenide layer is reduced because of the resulting increase in the concentration of HCl. The reduction in the buffer layer growth rate increases out-diffusion of impurities from the substrate into the buffer layer with the previously mentioned undesirable reduction in resistivity of the buffer layer.